1. Field of the Invention
The present invention relates to a particle separator, especially a particle filter, for the separation of particles from the exhaust gas stream of an internal combustion engine.
2. Description of the Related Art
According to the present invention, the term “particle separator” is understood as a type of particle separator with flow channels, which are significantly larger in diameter than the largest occurring exhaust gas particle. Particle separator also covers what are usually called particle filters, which have flow channels with diameters in the range of the diameters of the particles present in an exhaust gas stream and/or in the case of which the exhaust gas flows through a filter medium. Particle filters are subject to clogging, which has the effect of increasing the backpressure—an effect that gets worse as the quantity of deposited soot increases—and also lowers engine power.
A known way of regenerating a particle filter consists oxidizing the carbon-containing soot present in the particle filter. For this purpose, the exhaust gas temperature is actively raised to more than 550° C., as a result of which it becomes possible for oxidation to occur with the residual oxygen present in the exhaust gas. The exhaust gas temperature is increased by the addition to the exhaust gas of hydrocarbons, which are oxidized on a catalyst installed upstream of the particle filter. The heat liberated by this reaction leads to the required increase in temperature, which leads in turn to the oxidation of the soot deposited at the inlet to the filter according to the following equation:C+O2→CO2 
Because this reaction proceeds exothermically, the exhaust gas becomes even hotter, so that the exhaust gas temperature rises continuously as the gas proceeds toward the filter outlet. Because the regeneration must be actively induced, this is so-called “active” filter regeneration.
Another way of regenerating a particle filter is known from EP 0 341 832 A2. Here the nitrogen monoxide (NO) present in the exhaust gas is oxidized to nitrogen dioxide (NO2) in an oxidation catalyst upstream of a particle filter with the residual oxygen also present. This nitrogen dioxide reacts in turn in the particle filter with the carbon particles to form CO, CO2, N2, and NO. This is called so-called “passive” regeneration.
To improve soot burn-off, particle filters are being provided with a catalytic coating for the oxidation of nitrogen monoxide. These catalysts usually contain platinum. The disadvantage here is that the nitrogen dioxide formed on the particle filter can be used only to oxidize particles deposited downstream of the catalytically active layer for nitrogen monoxide oxidation within the filter medium. If a layer of deposited particles, a so-called “filter cake”, forms on the surface of the filter and therefore on the catalytically active layer, the nitrogen monoxide oxidation catalyst then lies downstream of the filter cake, which means that the soot particles deposited there cannot be oxidized with the help of nitrogen dioxide coming from the nitrogen monoxide oxidation catalyst applied to the particle filter. For these reasons, in spite of the catalytic coating on the particle filter, it is impossible to dispense with a nitrogen monoxide oxidation catalyst upstream of the particle filter, and this results in a structure with a relatively large overall volume.
Additives such as iron and/or cerium, which lower the ignition temperatures to approximately 350° C., can be used to regenerate the particle filter.
Common to all of the methods or systems described is that, if it is not possible to oxidize all of the carbon deposited in the particle filter, the amount of carbon and thus the exhaust gas backpressure will increase continuously. When a certain critical mass is reached, it is possible at high exhaust gas temperatures for the carbon to ignite in an uncontrolled manner; the carbon then burns up very quickly in the oxygen. This causes the temperature to increase to as much as 1,200° C., which damages the particle filter. As previously described in connection with active regeneration, the maximum temperatures are usually at the outlet of the filter.
Such damage can be almost completely avoided by the use of more heat-resistant materials such as silicon carbide or aluminum titanate in place of the normally used cordierite. Nevertheless, these highly heat-resistant materials are very difficult to process, very expensive, and also heavier than the conventional materials. Another disadvantage of these highly heat-resistant materials is that their heat capacity is usually greater than that of, for example, cordierite. This greater capacity to store heat means that temperature peaks are capped. This is critical, especially in the case of active filter regeneration, because more heat must be supplied to achieve the ignition temperatures at the filter inlet in spite of the buffering effect of the filter substrate with its high heat storage capacity.